1. Field of the Invention
This invention relates to a method and apparatus to improve the quality of paper made on a Fourdrinier paper making apparatus.
More specifically, the invention relates to apparatus for "shaking" the forming fabric of a paper making machine, that is, providing cross machine motion to the fabric in the wet forming section or wet end of the machine as it is commonly known.
2. Description of the Prior Art
On Fourdrinier type paper making machines, stock, made up of fibers and fillers in an aqueous solution containing generally about 99.5% water, is fed onto an endless moving Fourdrinier forming screen and water is drained from the stock through the screen by means of dewatering devices consisting of foils, table rolls and suction boxes, leaving the fibers in a mat or web form on top of the screen. The fibrous mat is then peeled from the forming screen continuously and transferred to other parts of the paper machine where it is pressed and dried.
In spite of attempts to thoroughly mix the stock before it is deposited on the forming screen, the fibres of the stock invariably tend to agglomerate in the head box and become deposited on the screen in clumps or flocs so that the paper will not be of uniform density and, even if the stock is deflocculated in the head box, it will, if not distributed, reflocculate in a short time while being formed. Also, there is a tendency for stock fibres to become aligned in the machine direction which is detrimental to cross machine strength of the paper.
In early designs of the Fourdrinier machine a "shake" was utilized to agitate the stock at the wet end on the endless screen to continuously deflocculate the fibres of the stock to improve the formation of the paper. Formation, relating to paper making, is defined in "The Dictionary of Paper" -- published under the Auspices and direction of the American Paper an Pulp Association, New York, 1940, George Banta Publishing Company, Menasha, Wis., as a property which is determined by the degree of uniformity of distribution of the solid components of the sheet with special reference to the fibers. It is usually judged by the visual appearance of the sheet when viewed by transmitted light. Formation is very important, not only because of its influence on the appearance of the paper, but because it influences nearly all other properties. The "shake" is a transverse oscillation of the wet end of the Fourdinier section in the plane of the screen and at right angles to the direction of its travel. Thus, the stock and fabric, in addition to moving in the longitudinal direction, are subjected to a small transverse oscillatory motion as they travel down the machine.
Since the stock is fluid, a shearing motion is set up between the stock and the screen which helps to break up flocs and aids in paper formation. In its original form, the "shake" motion was imparted to the wet end of the forming screen by shaking the breast roll and the table rolls with their mounting frames and bearings. As Fourdrinier machines were made larger, it became increasingly difficult to shake the whole wet end in this manner because of the increased mass of the machine. In addition, as the speed of the machines increased, there was a corresponding need to increase the frequency of the shake. The increased mass, coupled with the need to increase the frequency of shake with speed, caused the required shaking force to become very high and practically impossible to achieve. In practice, most paper-makers do not attempt to shake the stock when speeds over 1000 fpm are reached. As a result, paper made at speed over 1000 fpm suffers in formation somewhat compared with paper made at lower speeds with the help of shake.
As Fourdrinier paper machine were speeded up beyond 1000 fpm, the action of the "shake" was provided to some extent by a vertical vibration of the screen which is caused by a sucking action of the table rolls and foils acting on the flexible screen. These suction forces increased rapidly with speed and the vertical deflection of the screen in the down-stream nips of the table rolls or foils become large enough to agitate the stock. This type of agitation is well described in the Canadian patent of Wrist, 586,545, as "stock kick-up" as well as in many other places in paper-making literature.
In the present state of the art, a modern Fourdrinier machine contains mostly foils in the forming section of the machine and these foils are designed so as to achieve the best combination of drainage, retention and stock kick-up so as to produce the best quality of paper. Factors such as the foil angle, the length of the foil surface in the machine direction and the spacing between adjacent foils are critical in achieving the optimum result. When properly adjusted and designed, fairly good quality paper can be made and, for this reason, the need for a shake on high speed machines is overcome to some extent. However, the proper adjustment of a foil table requires great skill and inevitably some compromise must be made. Also, if the quality of the stock changes or the machine is speeded up, a different table configuration is theoretically required for optimization.
One attempt to overcome the problem of having to shake a large mass of machinery was to provide special table rolls which could move axially through their bearings at the ends. In this way, only the smaller mass of the breast roll and the shells of the table rolls need be shaken. (Pulp and Paper Manufacture Vo. 3 (1953) pp 142 Fourdrinier Shakes -- J. Newell Stephenson). This method never came into wide-spread use because of mechanical difficulties.
In a further attempt to overcome the problem of having to shake a large mass, U.S. Pat. No. 1,839,158, McDonnell, teaches a method of shaking only the breast roll and selected table rolls or only selected table rolls in an attempt to localize the shake in a critical area. However, even the selected table rolls have a mass of the order of hundreds of pounds, and the inertia presented by so large a mass prevents the rolls from being shaken at a high enough frequency as required for high speed machines. In addition, because of the cylindrical shape of the roll, only a very small surface of the roll is in contact with the upper run of the Fourdrinier screen so that the frictional engagement between the roll and the screen is not very good. To present a larger area of contact, McDonnell proposes an upward slope of the Fourdrinier screen in the area of a single movable table roll. Because of these problems, the method and apparatus proposed in the U.S. Pat. No. 1,839,158 has never come into wide-spread use.
The prior art has also taught electric vibrators oscillating the Fourdrinier screens in a vertical plane (U.S. Pat. No. 1,841,702 -- E. E. Berry). This method has no great advantage over the present state of the art where vertical vibrations are caused by foils or table rolls. None of the previous attempts have come into wide-spread use for speeds over 1000 fpm, thus leaving a wide range of speeds without an effective means of stock oscillation or shaking. Most modern paper making machines are operated at speeds in excess of 1000 fpm and many are operated in the range of 2,000 - 3,000 fpm.